The present invention relates generaly to CRT electron beam defletion systems, and more particularly to the compensation of delays inherent in electromagnetic deflection systems.
Television and other CRT-type image display systems typically use a large magnetic yoke to scan the CRT's electron beam(s) over the surface of its phosphor display screen in a raster pattern. Most multicolor cathode-ray tubes have three closely-spaced beams, which must be substantially coincident, or converged, at the screen and remain so as they are deflected over its surface. Thus, in addition to deflection by the common magnetic yoke to produce a scanned raster, each beam is also deflected individually as needed to maintain convergence over the entire screen. This small angle, variable beam deflection, referred to as dynamic convergence, is accomplished by applying correction signals to convergence coils on the neck of the CRT.
There is an inherent delay (not a pure delay, but instead a complex error function the major part of which is a time shift) between the input current waveform driving a deflection coil and the flux it produces. In a television-type display, the corrections required for beam convergence and for pincushion distortion and tilt of the vertical raster can be significantly affected by such delays. Most such displays operate at fixed, relatively low scan rates--approx. 15 KHz for U.S. commercial television--at which delays can be tolerated, or compensated using standard controls. However, electromagnetically-deflected CRT displays operating at higher horizontal scan rates encounter a much more severe problem, particularly those designed to operate over a wide range of frequencies. For example, a delay of six percent of the horizontal width at 60 .mu.s active time (approx. 15 KHz) would increase to 25% of the horizontal width at 14 .mu.s active time (approx. 50 KHz). Video type display devices operating over a wide range of horizontal scan rates tus require some means of compensating for the effects of deflection system delays on beam position-related correction signals, such as those for beam convergence or top and bottom raster tilt.